1. Field of the Invention
The present invention relates to a compound semiconductor device, and more particularly to a compound semiconductor device which is improved in isolation thereof.
2. Description of the Related Art
A monolithic microwave integrated circuit (MMIC) of a compound semiconductor device uses microwaves having a GHz band in many cases, thus resulting in an increasing demand for improvement in isolation.
For example, in a case where two elements (any of an FET, a wiring, an electrode pad, and an impurity region) are placed in proximity to each other in the same chip, a technology, in which a high concentration impurity region is placed between the two elements, is known. The high concentration impurity region improves the isolation between these two elements either by using a floating potential or by applying a GND (ground) potential. This technology is described for instance in Japanese Patent Application Publication No. 2004-134589.
As described in the patent application publication, when the high concentration impurity region to which a GND potential is applied is placed between the two elements (for example, two adjacent FETs) in proximity to each other, high frequency signals which transmit in the FET leak to the GND potential in some cases.
That is to say, carriers drift by the bias between the two FETs and the high concentration impurity region being at the GND potential so that the high frequency signals which transmit in the two FETs leak via the high concentration impurity region to the ground. Accordingly, there has been a problem where an increase in insertion loss is observed in a case of switch MMIC, for example.
On the other hand, when the high concentration impurity region in an electrically floating state in potential is placed between two elements (for example, FETs) in proximity thereto, the small amplitude high frequency signals leaking from the FET to a substrate can be absorbed by the high concentration impurity region, thereby resulting in improved isolation between the two FETs.
However, when large amplitude high frequency signals leak from the FET to the substrate, the potential itself of the high concentration impurity region in the electrically floating state in potential varies. This is because the power of the leaked high frequency signals is so large that the potential of the high concentration impurity region varies. As a result, there arises a problem that sufficient isolation cannot be ensured between the two elements such as the FET, and the like.